1. Field of the Invention
This invention relates generally to a wireless communication system, and, more particularly, to wireless communications s.
2. Description of the Related Art
The list of devices that use wireless communication techniques to transmit voice and data signals has expanded dramatically in recent years to include, among other things, cell phones, personal data assistants, global positioning system receivers, laptop computers, and desktop computers. And the number of devices on the list, as well as the services they are likely to provide, is expected to continue to grow. The proliferation of wireless communication systems has led users to expect to access these systems at virtually any time and in virtually any place. However, before any voice or data information can be transmitted via a wireless communication system, the wireless device (sometimes referred to as a user equipment, or UE) and a base station (sometimes referred to as a Node-B) should establish a communication channel.
To establish the communication channel, the UE typically transmits a known sequence, such as a sequence containing a series of symbols, on an access channel that is monitored by a receiver at the Node-B. The Node-B receiver detects the known sequence and uses it for functions such as estimating the round-trip time delay between the UE and Node-B. For communications systems governed by the Universal Mobile Telecommunications System (“UMTS”) standard, a random access transmission procedure may be employed so that multiple UEs can share the same physical resources when establishing communication channels with a Node-B of a given cell. The Random Access Channel (RACH) is a common uplink physical channel that carries one or more preamble sequences and one or more message parts.
The random access transmission includes a RACH preamble transmission followed by a message. For example, in a communication system governed by UMTS, a RACH preamble transmission may be 4096 chips long and consist of 256 repetitions of length 16 Walsh-Hadamard preamble sequence signatures. The Node-B receiver may detect the RACH preamble by correlating the received signal with a scrambling code and an expected signature sequence. Since the received preamble signal is delayed by the round-trip propagation time between the Node B and the UE, with respect to Node B transmit time, the Node-B receiver searches over a time range, or search window, corresponding to an expected range of possible round-trip delays between the Node B and the UE, which may be located anywhere within the cell served by the Node-B. For example, the Node-B receiver may search a 1280-chip search window, which may correspond approximately to the possible locations of the UE within a cell having a 50-kilometer radius around the Node-B. A preamble is detected and the signature sequence corresponding to a transmitted signature is found when the correlation energy exceeds a certain predefined threshold.
The resolution of the time search performed by the Node-B receiver to detect the RACH preamble is coarse, e.g. at a half-chip resolution, which is not typically accurate enough to permit synchronization of the UE and Node-B signals. Accordingly, the Node-B may also include a multipath searcher, which may determine an increased-accuracy round-trip time delay that may allow synchronization in a time-varying mobile channel environment. For example, the multipath searcher may determine a power delay profile by searching for pilot sequence and/or one or more control bits in a 128-chip window centered on the round-trip delay time estimated using the RACH preamble. The increased-accuracy round-trip delay time and/or power delay profile may then be used to assign the appropriate RAKE fingers. However, conventional multipath searchers do not utilize or provide spatial information, such as an angle of arrival of the signal from the UE to the Node-B. Thus, conventional multipath searchers may not be able to take advantage of many features of beam-forming antenna systems, such as increased transmission accuracy, sensitivity, power, and the like. Moreover, the failure to use spatial information may reduce the accuracy of the round-trip delay time and/or power delay profile in regions characterized by channels with small angle spread.
The present invention is directed to addressing the effects of one or more of the problems set forth above.